Liquid systems and solid systems are the two basic types of rocket propulsion systems that are generally used in the rocket industry. In a solid propellant system, solid rocket fuel and an oxidizer are mixed together and allowed to cure inside a rocket case to form a solid propellant material, which is then ignited in the rocket case. Upon ignition, pressure forms within the rocket case and gases are released through a nozzle to produce thrust. In a solid propellant system, the solid propellant burns uninterrupted until all the propellant is exhausted, which can be undesirable in certain circumstances. Solid systems can be quite complicated, and are subject to several requirements during manufacture in order to minimize safety risks during use. For example, the solid propellant must be crack-free, as propellant grains which contain cracks present a risk of explosive failure of the vehicle. Solid propellant systems can be inadvertently ignited by mechanical shock and static electricity. Consequently, the manufacturing process requires extreme safety precautions, which increases manufacturing costs.
In a liquid system, a liquid oxidizer is fed into a combustion chamber in combination with a liquid fuel. The oxidizer and liquid fuel are mixed in the combustion chamber, where they react to produce gases under high temperature and high pressure. The gases exhaust through a nozzle from the combustion chamber to thereby produce thrust. Although widely used, there are certain drawbacks associated with liquid propulsion systems. One such drawback is that the mixing of reactants requires a high performance pressurization system for the fuel and oxidizer, which can contributes to a high cost with respect to both money and maintenance. Like solids, a liquid system can also explode since the oxidizer and fuels can be inadvertently mixed together. Another drawback is that exotic—and therefore expensive—materials must be used for the various components of the system, which increases the monetary cost of the systems.
Another type of rocket propulsion system are the hybrid systems, which are generally not as widely used as liquid and solid rocket fuel systems. A hybrid system combines aspects of both liquid systems and solid systems in that one propellant is stored as a solid and another propellant is stored as a liquid. In a typical system, the solid material is used as the fuel and the liquid material is used as the oxidizer. A variety of materials can be used as the fuel, including Plexiglas (polymethyl methacrylate (PMMA)), high density polyethylene (HDPE), hydroxyl terminated polybutadiene (HTPB), etc. Nitrous Oxide is a commonly used as the oxidizer, although other oxidizers can be used.
Hybrid systems have characteristics that can be highly desirable for certain situations and uses. For example, a hybrid system generally has higher specific impulse than solid systems. Specific impulse is the change in momentum per unit mass for the rocket fuel. Thus, a hybrid system can generate a high level of “push” for each unit of fuel that is used. Another advantage associated with hybrid propulsion systems is the complete separability of the fuel from the principal oxidizer. This inhibits the potential for inadvertent ignition or catastrophic failure so that hybrid systems are inherently immune to inadvertent explosion. Yet another advantage is that hybrid systems have the ability to easily start, stop, and restart the combustion of the rocket fuel.
There is currently a need for improved hybrid propulsion systems.